Context. The diffuse gas in and nearby the Milky Way plays an important role in the evolution of the entire Galaxy. It has a complex structure characterized by neutral, weakly and highly ionized gas.
Aims: We probe this gas through the observation of its absorption lines in the high-energy spectra of background sources.
Methods: We used high-quality spectra of AGN Mrk 509, located at high Galactic latitudes obtained with XMM-Newton, HST and FUSE. We used advanced absorption models consisting of photo- and collisional-ionization.
Results: We constrain the column density ratios of the different phases of the interstellar medium (ISM) and measure the abundances of C, N, O, Ne, Mg, Al, Si, S, and Fe. We detect seven discrete interstellar clouds with different velocities. One is a typical low-velocity cloud (LVC) and three belong to the family of the intermediate-velocity clouds (IVCs) found near the Galactic disk. These four clouds show large deviation from Solar abundances in the gas phase, mostly caused by dust depletion. The other three clouds are ionized high-velocity clouds (HVGs) and are located either in the Galactic environment or in the Local Group halo as suggested by the signatures of collisional ionization. The similar abundances and ionization structure of the HVCs suggest a common location and origin: they might belong to the remainder of an extragalactic cloud which was captured by the Galaxy.
Conclusions: We have shown that combined UV/X-ray spectroscopy is a powerful tool to investigate the ISM. In common Galactic clouds, like LVCs and IVCs, the ISM shows a complex structure consisting of at least three different temperature phases.
